1. Field of the Invention
The present invention relates to a process and a device for the combustion of sulphur and/or sulphur-containing compounds such as, e.g., hydrogen sulphide, with the formation of sulphur dioxide.
2. Description of Related Art
In the prior art, various devices and processes are described for burning sulphur or hydrogen-sulphide-containing gases.
The combustion of elemental sulphur plays an important role, for example, in the industrial production of sulphuric acid.
The combustion of hydrogen sulphide is of industrial importance, for example in the form of the Claus process or the Wet Sulphuric Acid (WSA) process.
To ensure complete combustion of sulphur to sulphur dioxide (SO2), combustion usually proceeds in stoichiometric oxygen excess. This superstoichiometric combustion is described, for example, in Ullmann's Encyclopaedia of Industrial Chemistry, 5th Edition, 1994, Vol. A25, pp. 574 ff. The combustion proceeds there in a horizontally arranged cylindrical furnace which has a refractory lining and on its end side comprises a centrally arranged burner system. The liquid sulphur is atomized and mixed with the combustion air. The combustion chambers are designed to be adiabatic, i.e. there is no removal of heat from the combustion chamber.
In general, it is true that with an oxygen excess, the adiabatic flame temperature increases with increasing concentrations of sulphur and oxygen in the starting materials. For combustion temperatures above 1100° C., the formation of what are termed thermal nitrogen oxides (NOx) greatly increases. In order to avoid the formation of thermal nitrogen oxides, in WO2007/090671A2, a two-step combustion is proposed. The sulphur in this case is all added at the end side of the combustion chamber. The addition of the combustion air is divided into two steps. In the first step, substoichiometric combustion of the sulphur proceeds, i.e. under an oxygen deficit. The reducing conditions prevent the formation of NOx. In the second step directly following the first step, by further addition of air the remaining amount of sulphur is subjected to afterburning in an oxygen excess.
In DE1948754A1, a two-step combustion is likewise proposed, in which the first step proceeds substoichiometrically, and the second step proceeds superstoichiometrically with respect to oxygen. The combustion proceeds adiabatically in steps 1 and 2. Between the steps, the gas mixture is cooled by way of a heat exchanger.
Usually, the adiabatic combustion chamber is connected to a waste-heat boiler via an opening. The reaction gas is first cooled in the waste-heat boiler, preferably with steam recovery (see, e.g., WO2007/090671A2).
The design criteria of conventional adiabatic combustion chambers for the combustion of sulphur are based on the flame length and the diameter of the flame. The combustion chamber is dimensioned in such a manner that complete exhaustive combustion of the sulphur to SO2 proceeds, and so no unreactive sulphur leaves the combustion chamber and can enter the downstream flue gas cooling (WO2007/090671A2).
Exhaustive combustion is customarily ensured by correspondingly long residence times with a sufficiently high temperature in the adiabatic combustion chamber. This requirement determines the size of the combustion chamber and the temperature profile (which is virtually constant downstream of the flame in the case of adiabatic combustion chambers). The requirement for complete exhaustive combustion is generally justified by the statement that otherwise damage would be feared in the region of the boiler (see, e.g., WO1995/32149A1 or WO2007/090671A2).
EP1295849B1 is concerned with the low-temperature combustion of sulphur in air. Special measures are taken in order not only to eliminate amounts of NOx already present in the starting material, but also to avoid amounts of thermal NOx formed by combustion in air. All of the combustion air is added at the bottom of a combustion chamber and the sulphur is fed in a plurality of, preferably two, steps, perpendicular to the direction of flow of the combustion air. The combustion temperature is kept in a range between 500° C. and 700° C. Sulphur is atomized via a plurality of special pulsed fan nozzles distributed peripherally on the circumference of the combustion chamber perpendicularly to the flow of the low-turbulence combustion air flowing in at the same velocity. Between the combustion zones, heat uncoupling proceeds. Only combustion with air is described, optionally with addition of SO2/NOx-containing gases.
DE4002465A1 describes that amounts of NOx already present in the reaction gas can be reduced by addition of sulphur in an adiabatic combustion chamber.
Using air which contains 20.95% by volume of oxygen, with stoichiometric combustion of sulphur, theoretically an SO2-containing gas having a maximum of 20.5% by volume of SO2 may be obtained. The largest component of the SO2-containing gas is nitrogen. Usually, the resultant SO2 is further oxidized to SO3 in a catalytic process in order to obtain sulphuric acid. A high nitrogen fraction in the SO2-containing gas is unfavourable, inter alia, because it leads to large apparatus dimensions and high fan outputs.
The current contact processes in addition permit gases having a sulphur dioxide concentration of up to 66% by volume to be fed in (WO2008/052649A1).
With regard to reducing the fraction of unwanted nitrogen in the product gas of the combustion and with regard to an efficient and compact plant for further processing the sulphur dioxide produced (e.g. to form sulphuric acid), it would therefore be desirable to carry out the combustion in a combustion gas that is enriched with oxygen compared with air. The combustion of sulphur and sulphur-containing compounds such as, for example, hydrogen sulphide, in an oxygen-rich combustion gas, however, causes some technical problems.
Increasing the oxygen concentration causes an increase in the adiabatic flame temperature. High flame and flame point temperatures can lead to damage (burning off) of the nozzles and increase the formation of NOx in the flame.
In addition, damage to the combustion chamber wall, which usually contains iron, must be feared. In order, for example, to prevent the formation of iron(II) sulphide FeS, iron(II) disulphide (FeS2) or iron(III) sulphide (Fe2S3), the combustion chamber wall temperature should be below 480° C. On the other hand, the combustion chamber wall temperature should be above the dew point of sulphuric acid. The sulphuric acid dew point is dependent on the SO3 concentration and the H2O concentration in the flue gas, wherein the SO3 concentration in turn is dependent on the concentrations of SO2 and O2, and also on the temperature and residence time, wherein catalytically active substances can also be of importance.
As already discussed, in addition, at relatively high temperatures, the formation of thermal NOx from molecular nitrogen (N2) must be expected. The formation of thermal NOx is dependent, inter alia, on the temperature and the residence time. At high temperatures and a long residence time in the combustion zone, intensified nitrogen oxide formation proceeds. Since in the adiabatic combustion chambers customarily used for the combustion of sulphur, owing to the required exhaustive combustion, a long residence time is required, these types of combustion chamber are not efficient.
In addition to this thermal NOx, on combustion of sulphur-containing compounds from organic sources, the formation of NOx from chemically bound nitrogen (e.g. in ammonia or amines) can also be significant.
For reducing the temperature in the combustion chamber, it is proposed in Laid-Open Application DE10351808A1 to recirculate some of the resultant SO2 for dilution of the reaction mixture. The combustion proceeds adiabatically, i.e. the temperature limitation is achieved solely by dilution of the combustion gas with SO2. In the process resulting from this proposal, approximately 80% by weight of the SO2 exiting from the combustion chamber is circulated as cycle gas. Owing to the high fraction of the cycle gas, high outputs of the fan and large apparatus dimensions and piping cross sections are necessary, which leads to an uneconomic process.
In U.S. Pat. No. 3,803,297, a process is described for producing sulphur trioxide and sulphuric acid, in which sulphur is first burnt with technical-grade oxygen in a multiplicity of successively following combustion steps to form SO2. In the individual combustion steps, the combustion proceeds adiabatically. Between the combustion steps, cooling is performed. In the first combustion step, 5% to 40% of the total amount of the sulphur that is to be oxidized is mixed with half to all of the molar amount of recirculated sulphur trioxide. The addition of sulphur trioxide effects a cooling in the combustion. The high combustion temperatures during the combustion of sulphur effect a decomposition of sulphur trioxide to sulphur dioxide and oxygen with takeup of heat. However, recirculating sulphur trioxide that has already been obtained is inefficient from economic aspects. In addition, the adiabatic combustion is unfavourable, since the heat development can only be controlled via the gas composition. Since the purpose is to burn the starting materials at high concentrations, a correspondingly high number of intermediate cooling stages is necessary between the combustion steps, in order to limit the temperatures to a required maximum value of 2000°.
The combustion of sulphur-containing compounds, in particular of hydrogen-sulphide-containing compounds, in addition, plays an important role in the workup of off-gases from the refinery industry. Fossil fuels, such as natural gas, coal, oil sand, oil shale and petroleum contain organic and inorganic sulphur compounds.
The removal of these sulphur compounds is necessary not only for the further processing of the raw material but serves especially for reducing air impurities. The legal requirements for the residual amounts of sulphur compounds in off-gases have increased considerably in recent years.
For the removal of sulphur compounds from fuels and combustion products, a multiplicity of physical and chemical conversion processes exist.
In the case of solid fuels, the sulphur compounds, after the combustion in the power plant, are absorbed as sulphur dioxide by a flue gas desulphurization, e.g. by means of milk of lime, and converted into calcium sulphite. By oxidation with residual oxygen present in the off-gas, the end product gypsum is formed.
In the case of liquid fuels (e.g. diesel fuel or light heating oil), maximum permissible sulphur contents are prescribed. These fuels are desulphurized as early as in the refineries. The sulphur compounds present in the crude oil are recovered after the distillation, wherein what is termed the heavy oil fraction has the highest sulphur concentrations. Desulphurization customarily proceeds using gaseous hydrogen. The organic sulphur compounds are converted into hydrogen sulphide in this process. Then, the hydrogen sulphide is removed, for example by means of an amine scrubber. In the amine scrubber, the hydrogen-sulphide-containing process gas termed acid gas (or sour gas) is bound to an amine in an absorber. A gas substantially freed from H2S (sweet gas) and an H2S-loaded amine solution are formed. In the regenerator, thermal separation of the acid components from the amine proceeds, in such a manner that the amine can again be used for scrubbing. The concentrated hydrogen-sulphide-containing gas has H2S concentrations up to 90% by volume. This gas is customarily used for obtaining sulphur by the Claus process.
In the petrochemical industry, in addition, what is termed Sour Water Stripper Gas (SWSG) arises, which has a composition of roughly equal molar fractions of hydrogen sulphide, water and ammonia.
Also, in the combustion of coal or heavy oil in power plants in which the fuel is gasified in advance in an oxygen deficit, a hydrogen-sulphide-containing synthesis gas is formed which is purified before the combustion.
Hydrogen sulphide, in addition, is present in differing concentrations in petroleum-associated gas and in natural gas having a fraction of up to 30% by volume, and in the off-gas of sewage treatment plants having a fraction of up to 5% by volume.
The industrial use of hydrogen sulphide is limited. Therefore, it is usually first converted into elemental sulphur and then into sulphuric acid.
If the hydrogen sulphide is present in the off-gas in concentrated form, i.e. at contents of greater than 20% by volume, the production of sulphur by what is termed the Claus process is economic.
The Claus process comprises the following reaction steps:2H2S+3O2→2SO2+H2O2H2S+SO2→3S+2H2O
Industrially, sulphur generation proceeds by the Claus process in a plurality of steps. In a first step, H2S is burnt to SO2 in a combustion apparatus. A majority of the resultant SO2 already reacts in the combustion apparatus with remaining H2S to form sulphur. After separating off the sulphur, one or more catalytic steps follow for the further reaction of H2S with SO2.
In the Claus process, maintaining the stoichiometry is important, since otherwise excess SO2 or H2S can pollute the environment.
In addition, the reaction of H2S with SO2 in the Claus process proceeds in an incomplete manner. In order to increase the degree of sulphur retention further, various processes have been developed which free the off-gas from sulphur compounds downstream of the catalytic steps. The best known is what is termed Shell Claus Off-gas Treating (SCOT) process.
The majority of the sulphur obtained from the H2S-containing off-gases is further processed to sulphuric acid. Therefore, under some circumstances, it is advantageous to burn hydrogen sulphide directly to form sulphur dioxide and then immediately further oxidize it to sulphur trioxide and not to follow the detour via sulphur (Claus process).
What is termed the Wet Sulphuric Acid (WSA) process converts hydrogen-sulphide-containing off-gases directly to sulphuric acid. In this case the hydrogen-sulphide-containing gas is first burnt and the water- and sulphur-dioxide-containing gas is then fed to a catalytic oxidation. The resultant SO3 reacts with the water present to form gaseous H2SO4. Liquid H2SO4 is then obtained by condensation of the gaseous sulphuric acid. The combustion of H2S proceeds with air in the uncooled, i.e. adiabatic, combustion furnace (Sulphur 312, September/October 2007, pages 80-85).
In addition to hydrogen-sulphide-containing gases, in the petrochemical industry a number of further sulphur-containing off-gases also arise, which likewise must be treated on account of the environmental guidelines (e.g. German Federal Air Pollution Prevention Law).
Those which may be mentioned are, for example, sulphur-containing off-gases from calcination processes which have a relatively high concentration of SO2 (up to 10 000 ppm).
In addition, high amounts of flue gases occur in combustion processes, which likewise can have considerable concentrations of SO2. These are customarily treated to date according to the abovementioned flue gas desulphurization, e.g. with milk and lime, in which large amounts of gypsum are produced.
The off-gas arising in the petroleum-processing industry in the regeneration step of a fluid catalytic cracker, in addition to significant fractions of SO2, contains, as sulphur-containing compound, further fractions of oxidizable components such as carbon monoxide and also relatively high fractions of nitrogen oxides, and so here also afterburning is required.